The AMP-activated protein kinase (AMPK) acts as an intracellular metabolic sensor in a variety of cells, where it monitors and responds to variations in the AMP:ATP ratio (Hardie et al., Annu. Rev. Biochem. 67:821-855, 1998). Upon activation of AMPK, the enzyme phosphorylates a number of protein substrates to decrease further ATP usage by the cell. AMPK is a heterotrimeric complex consisting of a catalytic subunit (α) and two associated subunits (β and γ). Both the β and γ subunits are required for optimal activity of the α catalytic subunit. The AMPK complex is evolutionarily conserved and also can be found in yeast and plants. Mammalian AMPK is composed of different isoforms of subunits: α1, α2, β1, β2, γ1, γ2, and γ3 (Hardie and Hawley, BioEssays 23:1112 1119, 2001). Different combinations of isoform subunits are activated differently in vivo, and most likely also differ in substrate utilization. AMPK activity is found in all tissues, including liver, kidney, muscle, lung, and brain (Cheung et al., Biochem. J. 346:659-669, 2000).
AMPK is recognized as a major regulator of lipid biosynthetic pathways due to its role in the phosphorylation and inactivation of key enzymes such as acetyl-CoA carboxylase, fatty acid synthase (Hardie and Carling, Eur. J. Biochem. 246:259 273, 1997). More recent work has suggested that AMPK has a wider role in metabolic regulation (Winder and Hardie, Am. J. Physiol. 277:E110, 1999); this includes fatty acid oxidation, muscle glucose uptake, expression of cAMP-stimulated gluconeogenic genes such as PEPCK and G6Pase, and expression of glucose-stimulated genes associated with hepatic lipogenesis, including fatty acid synthase, Spot-14, and L-type pyruvate kinase. Chronic activation of AMPK also can induce the expression of muscle hexokinase and glucose transporters (Glut4), mimicking the effects of extensive exercise training (Holmes et al. J. Appl. Phiysiol. 87:1990 1995, 1999).
AMPK phosphorylates and modifies the activity of key enzymes of carbohydrate metabolism. In fact, AMPK plays an important part in lipogenesis, because it inhibits the synthesis of fatty acids and of cholesterol by inactivating acetyl-CoA carboxylase and HMG coreductase. AMPK reduces the expression of fatty acid synthase (FAS), which controls the synthesis of triglycerides. In addition, AMPK also reduces the expression of one of the key enzymes of gluconeogenesis (PEPCK), which manifests itself in inhibition of the hepatic production of glucose. AMPK increases the clearance of blood glucose by facilitating the transport of glucose to the skeletal muscle.
All those properties combine to make AMPK a target of choice in the treatment of diabetes and of the metabolic disorders associated therewith, the search for pharmacological activators of AMPK accordingly being of fundamental value to the treatment of those pathologies (Winder and Hardie, Am. J. Physiol. 277:E110, 1999).
Compounds such as 5-aminoimidazole-4-carboxamide-1(β)-D-ribofuranoside (AICAR) and metformin, have been shown to activate AMPK in vivo at high concentrations (Bergeron, R. et. al. Effect of 5-aminoimidazole-4-carboxamide-1(β)-D-ribofuranoside infusion on in vivo glucose metabolism in lean and obese Zucker rats. Diabetes 50:1076 (2001); Song, S. M. et. al. 5-Aminoimidazole-4-darboxamide ribonucleoside treatment improves glucose homeostasis in insulin-resistant diabeted (ob/ob) mice. Diabetologia 45:56 (2002); Halseth, A. E. et al. Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations. Biochem. and Biophys. Res. Comm. 294:798 (2002), Buhl, E. S. et al. Long-term AICAR administration reduces metabolic disturbances and lowers blood pressure in rats displaying feature of the insulin resistance syndrome. Diabetes 51: 2199 (2002)). Metformin increases AMP-activated protein kinase activity in skeletal muscle of subjects with type 2 diabetes. Diabetes, 51: 2074 (2002), although it has to be determined to what extent its antidiabetic action relies on this activation. As with leptin and adiponectin, the stimulatory effect of metformin is indirect via activation of an upstream kinase (Zhou, G. et. al. Role of AMP-activated protein kinase in mechanism of metformin action. The J. of Clin. Invest., 108: 1167 (2001)).
Not withstanding recent advances, the need still exists for more effective AMPK modulators.